A long-term goal of this grant application is to understand how selection of the T cell repertoire is regulated by the functions of the CD4 molecule. Mutations will be introduced into the CD4 molecule to determine the contributions that specific interactions and post-translational modifications make on its capacity to augment T cell antigen recognition and promote normal thymocyte development and T helper cell differentiation. Stage-specific gene expression systems (relying on the Cre recombinase) will be used to inactivate CD4 at key stages in development. These systems will permit an analysis of CD4 functions in peripheral T cells in the absence of confounding abnormal thymocyte development. The systems will be used to study the regulation of T helper cell survival and homeostatic proliferation by CD4 in addition to its role in immune responses and T helper cell differentiation. A novel system that permits inactivation (or activation) of genes in activated T cells will be used to study the functions of CD4 and p56Lck in memory T cells. This system will also be used to mark and study the properties of memory T cells. Finally, the application is also focused on the continued development of conditional gene expression systems through the creation of new cre and reporter strains of mice. These studies will contribute to a better understanding of the physiological significance of the CD4 molecule, and its regulation of T helper cell responsiveness, survival, and differentiation through enhancement of T cell receptor signal transduction. They will also lead to the creation and characterization of strains of mice with general applicability for use in the study of a broad range of immunological questions.